Brake fluid absorbs moisture from the air. The moisture mixes with the fluid. This lowers the boiling point, so under the heat of emergency braking the fluid can vaporize, your pedal goes to the floor, and you have no brakes. Also, the moisture-containing brake fluid can cause accelerated corrosion in the calipers and wheel cylinders.
I disagree with Bill...some DOT3 brake fluids hold their boiling point up better than the minimum requirement. Castrol LMA and Valvoline are two good ones.